1. Technical Field
The present invention relates to a common mode filter and its manufacturing method, and relates more particularly to a common mode filter that can be manufactured using a method of manufacturing printed circuit boards.
2. Background
Common mode filters are devices for suppressing common mode currents, which travel in the same direction in parallel wires, generally produce common mode radiation, and cause electromagnetic interference. To be used in portable communication devices, common mode filters need to be miniaturized and have highly compact structures. Thus, thin-film type common mode filters and multi-layer common mode filters are gradually replacing conventional wire-wound common mode filters. As their names imply, conventional wire-wound common mode filters include a cylindrical ferrite core and a wire around the ferrite core. The thin-film type common mode filter requires many semiconductor manufacturing processes to manufacture. For example, the coil of a thin-film type common mode filter is planar, which is formed on a plate-like ferrite using photolithography technique. In addition, the multi-layer common mode filter is formed with a plate-like ferrite, on which a coil is screen printed and sintered.
U.S. Pat. No. 7,145,427 B2 discloses a common mode filter that allows adjustability of the common mode impedance of the coil of a common mode filter. A coil is formed on a magnetic substrate and a portion of non-coil structures formed by filling a resin including magnetic powder into regions, on the magnetic substrate, from which the insulation layer has been removed. The magnetic substrates are then planarized and bonded to obtain the common mode filter.
U.S. Pat. Nos. 6,356,181 B1 and 6,618,929 B2 disclose multi-layer common mode filters. A coil is formed on a magnetic substrate and covered by magnetic material. In the two patents, different wire layouts of the coils are proposed for decreasing impedance for differential signals. However, because the wire layouts are continuous and distributed in different layers, the common mode filters are complicated, requiring more control factors.
Thus, the market needs a common mode filter that can be easily manufactured and is compatible with present manufacturing techniques. As such, the drawbacks of conventional common mode filters can be overcome and manufacturing costs can be reduced.